Conformations will be examined in detail in the next three sections. The compounds of living systems are tailor-made to perform highly specific or even unique functions by virtue of their particular configurations and conformations. Study of conformations and conformational equilibria has direct application to explaining the extraordinary specificity exhibited by compounds of biological importance. Constitutional isomers (isomers with different bond connectivity) arise in organic compounds as a result of differing carbon-carbon. Hassel’s work involved the physical determination of preferred conformations of small molecules, whereas Barton was the first to show the general importance of conformation to chemical reactivity. Barton (Britain), for which they shared the Nobel Prize in chemistry in 1969. Pioneering work in the field of conformational analysis was contributed by O. For instance, the isomer of 1,2-dichloroethene shown below has the trans configuration the isomer of 1,3-dichlorocyclobutane has the cis configuration: When referring to the configuration of a particular isomer, we mean to specify its geometry. The terms cis-trans isomerism or geometric isomerism commonly are used to describe configurational isomerism in compounds with double bonds and rings. Thus cis– and trans-2-butene are configurational isomers, as are cis– and trans-1,2-dimethylcyclopropane. Stereoisomers that do not interconvert rapidly under normal conditions, and therefore are stable enough to be separated, specifically are called configurational isomers. Interconversion of these isomers does not occur without breaking one or more chemical bonds. In the cis isomer, the methyl groups both are situated above (or below) the plane of the ring and in the trans isomer they are situated one above and one below, as shown in the figure. Ball-and-stick models of cis and trans isomers of 1,2-dimethylcyclopropane
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